The present invention is directed generally to radiocommunication systems and, more particularly, to techniques and structures for calibrating transceivers used in radiocommunication systems.
The first cellular mobile radio systems in public use were analog systems used to convey speech or other analog information. These systems comprised multiple radio channels for transmitting analog information between base and mobile stations by transmitting analog-modulated radio signals. More recently, digital systems have been implemented due to, for example, their promise of increased system capacity premised on their greater ability to tolerate interference. For example, in a time division multiple access (TDMA) radiocommunication system, each frequency can support a plurality of time-multiplexed channels, while in code division multiple access (CDMA) signals are encoded to such a degree that a high level of self interference is tolerable. Due to large existing customer bases having analog-only terminal equipment, dual-mode systems which support both analog and digital channels are becoming increasingly popular in certain areas. In the U.S., for example, systems specified by the EIA/TIA IS-54B publication are dual-mode systems.
In an analogous manner, digital signal processing (DSP) has become more prevalent in the radiocommunication industry for a variety of reasons. For example, unlike analog circuits, the operation of digital circuits does not depend on precise values of the digital signals. That is, since binary zeros and ones can be represented by significantly different voltages, the precision at which such voltages are held need not be very great. As a result a digital circuit is much less sensitive to the tolerances of component values and is also fairly independent of temperature, aging and other external parameters. The accuracy of digital circuits is thus much more reliable.
One part of the radiocommunication industry where the prevalence of DSP is having a significant impact is the design of radio transceivers. A transceiver, which is a combined transmitter and receiver, is used for transmitting and receiving signals over an air interface, e.g., between a base station and mobile station in a cellular system. Although the signals being transmitted and received are analog signals, the modulation may be of either analog or digital type, and the information carried by the signal will be digitally processed. Thus, a transceiver that uses digital signal processing techniques in a radiocommunication system is denoted a digital transceiver in this specification, although it may still contain analog parts, such as amplifiers and filters (and sometimes synthesizers and mixers), and regardless of whether the radio signals to be processed by the transceiver use analog or digital modulation.
In FIG. 1 a conventional digital transceiver 10 is shown schematically. Therein, a receive antenna 12 is used to capture signals transmitted over the air interface from, for example, other base stations and mobile stations (not shown). The received signals are input to analog receiver 14. Analog receiver 14 can include, for example, receive filters, downconverters and amplifiers for processing the signals received via antenna 12. However, many functions associated with processing received signals may not be included in block 14 as these functions are now performed using digital signal processing as described below. Accordingly, after being processed by the analog components of receiver 14, the output signal is provided to A/D converter 16 which transforms the analog signal into a digital signal. This allows digital computing part 17 to operate on the received signal and complete the signal processing which was not performed by analog receiver 14. For example, digital computing part 17 can be used to perform demodulation and decoding of the received signal.
On the transmitter side, digital computing part 17 performs various DSP routines which are used to prepare a signal for transmission, e.g., encoding and modulation. The digital signal output from digital computing part 17 is then input to a D/A converter 18 which transforms the digital signal into an analog signal. This analog signal is then received by analog transmitter 19 which includes various analog components that complete the signal processing for transmission by, for example, filtering, frequency upconverting and amplifying the signal before it is coupled to transmitting antenna 20.
Despite the fact that the number of analog components used in digital transceiver 10 have been reduced (i.e., by the substitution of DSP routines which perform signal processing tasks previously performed by additional analog components), the remaining analog parts (i.e., those denoted by blocks 14 and 19 in FIG. 1) continue to suffer from the imperfections described above. These imperfections result in gain and offset errors being introduced into the signals which are output from blocks 14 and 19.
To exemplify how such errors are introduced and how they are compensated according to a conventional solution, a model of an analog receiver will be studied in more detail. Those skilled in the art will appreciate that signals described herein are typically mathematically complex, i.e., with real and imaginary components. Accordingly, complex mathematics (e.g., complex-conjugation) is typically used to describe operations on these signals. However, to simplify this description the complex nature of signal operations is not shown explicitly in the equations and figures presented herein.
As mentioned above, an analog receiver normally suffers from a number of imperfections. Some of these imperfections create signal errors, such as gain errors and offset errors, that can be introduced to the wanted signal. FIG. 2 models how an analog receiver introduces an offset error and a gain error to a wanted signal S.sub.w. Therein, received signals are again captured by antenna 12. In the model of FIG. 2, ideal processing performed by receiver 14 is represented by block 22 which outputs the wanted signal S.sub.w, i.e., block 22 represents the effects of ideal analog components which have none of the imperfections described above. Signal processing blocks 24 and 26 represent the total offset and gain errors introduced onto the wanted signal S.sub.w by imperfections associated with the analog components of receiver 14. More specifically, block 24 represents the introduction of offset errors to the wanted signal and block 26 represents the effects of gain errors introduced by the analog components. The output signal, with introduced errors, is then represented by erroneous signal S.sub.o. Thus, the erroneous signal S.sub.o output by the receiver can be expressed as: EQU S.sub.o =(1+Gain.sub.eps)(S.sub.w +Offset.sub.eps)
where:
S.sub.w =Wanted signal PA0 S.sub.o =Erroneous signal PA0 Offset.sub.eps =Amplitude of the composite offset error PA0 Gain.sub.eps =Amplitude of the composite gain error
In a conventional receiver errors are typically compensated at the stage where they arise, using adjustment potentiometers and other adjustable analog components. FIG. 3 illustrates the principles of conventional calibration. Therein, the same reference numerals are used to denote the elements which were previously described with respect to FIG. 2. However, FIG. 3 also includes an offset compensation factor, Offset.sub.k, and a compensating gain factor, Gain.sub.k.
By adjusting the variable analog components to have values Offset.sub.k =Offset.sub.eps and Gain.sub.k =(1+Gain.sub.eps).sup.-1, the equality S.sub.out =S.sub.w will be achieved, whereby the receiver will be calibrated. The signal S.sub.out will then be fed through the A/D converter 16 to the digital computing part 17 for further processing.
As mentioned above, conventional calibration techniques rely upon the inclusion of adjustable components to compensate for errors introduced by analog components' imperfections. These adjustable components are used to realize the adjustments modelled by Offset.sub.k and Gain.sub.k. A more specific example of conventional calibration which illustrates this usage of adjustable components will now be described in terms of a transmitter portion of a digital transceiver. One example of an analog modulator for modulating analog data onto a carrier is the conventional quadrature modulator illustrated in the block diagram of FIG. 4. Quadrature modulators take advantage of the quadrature phases of sine and cosine waves to modulate twice the information on the radio carrier wave. For example, the even bits in a digital information datastream can be modulated on the cosine wave, and the odd bits in the digital information datastream can be modulated onto the sine wave.
In FIG. 4, the analog quadrature modulator includes an "in-phase" or I modulator 40, a "quadrature" or Q modulator 41, and a phase-splitting network 42 for supplying cosine and sine carrier frequency signals, respectively. Ideally, the signals provided by the network 42 are cos(.omega.t) and sin(.omega.t), where .omega. is the carrier signal's angular frequency. Also shown in FIG. 4 are an I and Q modulation generator 43 for supplying I and Q modulation signals, a combination network 44 for adding the outputs of the I modulator 40 and the Q modulator 41, and trim potentiometers 45, 46 for carrier balance/d.c. offset adjustments for the I and Q signals, respectively. Additional trim potentiometers 47, 48 for amplitude matching the I and Q signals, respectively, are also shown in FIG. 4. The phase-splitting network 42 may also be adjustable, as indicated by the diagonal arrows through trim potentiometers 47 and 48, to achieve as nearly as possible the desired 90.degree. phase difference between the sine and cosine carrier frequency signals.
The block diagram of FIG. 4 shows one exemplary way in which adjustable, analog components have been used to conventionally calibrate an analog device. However, these types of conventional calibration techniques rely upon the adjustment of potentiometers, capacitors and inductors during manufacturing. As will be recognized by those skilled in the art, this calibration process is both costly and unreliable. Moreover, there are certain types of errors which cannot be easily compensated for using these conventional techniques, e.g., in-band filter ripple. In-band filter ripple refers to the variation in the sensitivity of a receiver when tuning to different frequencies or channels within the transceiver's frequency range. This ripple is caused by the presence of band-limiting filters which are used in the receiver to remove strong, out-of-band signals. Unfortunately, in-band ripple can lead to a variety of difficulties, including inaccurate signal strength measurements.